In the field of photographic images composed of silver, the ratio of silver per unit area making up the image to the optical density of the image is generally called the covering power. The covering power is a measure for evaluating the optical efficiency of silver making up the image. The covering power of a silver halide photographic light-sensitive layer increases as the size of the silver halide grains decreases, and decreases as the size of the silver halide grains increases. On the other hand, the sensitivity of a silver halide emulsion layer increases as the size of the silver halide grains increase, and hence a silver halide emulsion having large grain sizes is generally used for high-speed photographic light-sensitive materials. Accordingly, a high-speed photographic light-sensitive materials requires a large amount of silver per unit area to obtain a definite image density. In other words, to obtain both high sensitivity and maximum image density, the photographic material must contain a larger amount of silver salt per unit area. This is the real circumstances of conventional high-speed photographic light-sensitive materials.
Their have been attempts at improving the covering power of silver halide photographic emulsion layers while maintaining high sensitivity. For example, techniques of incorporating various polymers in high-speed coarse grain silver halide emulsions are disclosed in British Pat. Nos. 1,048,057 and 1,039,471 and U.S. Pat. Nos. 3,043,697 and 3,446,618. Although these techniques may have some effect on improving the covering power the effect is insufficient and some of these techniques reduce the strength of coatings. In particular, when photographic materials having weak coating strengths are used in an ordinary automatic processor, part of the gelatin in the layers of the photographic materials is dissolved in a developer or a fix solution. The dissolved gelatin often sticks to fourth rollers of the automatic processor and is transferred onto the photographic materials causing stains on the photographic images.
On the other hand, U.S. Pat. Nos. 2,996,382 and 3,178,282 describe that by using a silver halide photographic material having surface latent image-type silver halide coarse grains and fine silver halide grains having fogging nuclei in the inside thereof in the same emulsion layer or in adjascent layers to each other, a photographic image having a high contrast and a high coverage power is obtained at a high sensitivity.
However, in these methods, it is impossible to obtain sufficiently high sensitivity, high density, and high contrast. Also, these conventional techniques are not desirable in that they take a long period of time for development at ordinary processing temperature and also a desired effect is not obtained by an ordinary high-temperature quick processing procedure.
In order to improve these faults, Japanese Patent Application (OPI) No. 89,749/82 (the term "OPI" indicates an unexamined publication of a patent application for open to public inspection) describes a technique of incorporating a cyclic compound including nitrogen having a thio ether compound (e.g., C.sub.6 H.sub.13 OCOCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 OCOC.sub.6 H.sub.13) or a thioketone group in a photographic material having a photosensitive silver halide emulsion and an internally fogged silver halide emulsion. The introduction of these additives in photographic materials are shown in the foregoing technique is desirable in that the developer for the photographic materials containing these additives can also be used for other photographic materials without reducing the shelf life of the developer as compared to the case of adding the additives to a developer. However, the foregoing technique is still insufficient for improving the faults described above.